1. Field of the Invention
The present invention relates to an inverter controller for driving a motor, and an air conditioner using it.
2. Description of the Related Art
FIG. 16 shows a general configuration of an inverter controller for driving a motor, used in general-purpose inverter or the like. As shown in FIG. 16, an inverter controller 100 for driving a motor includes a main circuit 102 and a control circuit 104. The main circuit 102 includes a DC power supply unit 106 and an inverter 108. The inverter 108 is connected to a motor 110. The DC power supply unit 106 includes an AC power source 112, a rectifier 114, a reactor 116 and a smoothing capacitor 118. The reactor 116 is a reactor for power factor correction which corrects a power factor of the AC power source 112. The smoothing capacitor 118 accumulates electric energy as a DC voltage source for the inverter 108.
On the other hand, the control circuit 104 includes a motor voltage command generator 120 and a pulse width modulation (PWM) controller 122. The motor voltage command generator 120 generates a voltage command value for each phase of the motor 110 on the basis of a motor speed command value ω* received from outside of the inverter controller. The PWM controller 122 generates PWM signals for the inverter 108 on the basis of the voltage command value for each phase of the motor 110 generated by the motor voltage command generator 120.
FIG. 17 is a graph to explain a relation between harmonic components of the AC power source current (indicated by current values) and the degree of the harmonic components relative to the AC power source frequency in the inverter controller 100 shown in FIG. 16. Here, an output voltage of the AC power source 112 is 220 V (AC power source frequency is 50 Hz), a power input to the inverter 108 is 1.5 kW, and a capacitance of the smoothing capacitor 118 is 1500 μF. In FIG. 17, the graph indicated by broken line refers to the relation when the reactor 116 has an inductance value of 5 mH, and the graph indicated by single dot chain line refers to the relation when it has an inductance value of 20 mH. The graph indicated by solid line refers to the standard of IEC (International Electrotechnical Commission). As shown in FIG. 17, when the reactor 116 has an inductance value of 5 mH, the third harmonic component largely exceeds the IEC standard. While, when it has the inductance value of 20 mH, the harmonic components up to the degree of 40 are below the IEC standard; the harmonic components up to the degree of 40 satisfy the IEC standard.
As indicated above, in order to satisfy the IEC standard when a load is high, in particular, the inductance value of the reactor 116 for power factor correction must be further increased. However, this brings about problems in that the inverter controller becomes increased in size and in weight, and its cost is also increased.
Accordingly, a DC power supply unit has proposed in which an increase of an inductance value of a reactor for power factor correction is suppressed, while the harmonic components of the AC power source current are reduced, and its power factor is high (for example, refer to JP laid-open patent publication No. 9-266674 (1997)).
FIG. 18 is a circuit diagram of such a DC power supply unit. In this DC power supply unit, an AC voltage Vin of an AC power source is applied to AC input terminals of a full-wave rectifier consisting of bridge-connected diodes D1 to D4, the output current of the full-wave rectifier is charged into an intermediate capacitor C by way of a reactor Lin, and an electric charge of this intermediate capacitor C is discharged into a smoothing capacitor CD, and a DC voltage is applied to a load resistance RL. This DC power supply unit further includes a transistor Q1, and a base drive circuit G1 for driving this transistor Q1. The transistor Q1 is connected to positive and negative DC current paths between the full-wave rectifier and the intermediate capacitor C, at the load side of the reactor Lin.
The DC power supply unit further includes pulse generation circuits I1 and I2 for applying a pulse voltage to the base drive circuit G1, and a dummy resistor Rdm. Each of the pulse generation circuits I1 and I2 has a circuit for detecting a zero cross point of the AC power source voltage, and a circuit for continuing to supply a pulse current to the dummy resistor Rdm from the time of detection of the zero cross point to the time when a momentary value of the AC power source voltage becomes equal to a voltage across ends of the intermediate capacitor C.
Here, the pulse generation circuit I1 generates a pulse voltage in first half of half cycle of the AC supply voltage, and the pulse generation circuit I2 generates a pulse voltage in second half of half cycle of the AC supply voltage.
When supplying a current forcibly to the reactor Lin by turning on the transistor Q1, a back flow prevention diode D5 is arranged so that the electric charge in the intermediate capacitor C may not be discharged through the transistor Q1, and further a back flow prevention diode D6 and a reactor Ldc for enhancing a smoothing effect are connected in series in the path used in discharging the electric charge of the intermediate capacitor C into the smoothing capacitor CD.
According to above-mentioned configuration, reduction of the harmonic components and heightening of the power factor can be achieved while preventing the device size from increasing, by turning on the transistor Q1 in part or all of a phase interval in which the momentary value of the AC power source voltage does not exceed the voltage across the ends of the intermediate capacitor C.
However, the conventional configuration as disclosed in, for example, JP Laid-open Patent Publication No. 9-266674 (1997) still requires the smoothing capacitor CD with a large capacitance, and the reactor Lin with a large inductance (JP Laid-open Patent Publication No. 9-266674 discloses results of simulation in the case of the smoothing capacitor CD's capacitance of 1500 μF and the reactor Lin's inductance of 6.2 mH) . Also it also includes the intermediate capacitor C, the transistor Q1, the base drive circuit G1, the pulse generation circuits I1 and I2, the dummy resistor Rdm, the back flow prevention diodes D5 and D6, and the reactor Ldc, so that it causes the DC power supply unit to become large in size, to need many parts and to rise in cost.